Conventional radio communication systems widely employ a scheme of intermittently transmitting a control signal such as system broadcasting information and paging information to mobile equipment at specified time intervals. The mobile equipment of such a radio communication system has a low-rate clock oscillator with small power consumption and low oscillation frequency such as an RTC (Real Time Clock), and a high-rate clock oscillator oscillating at a high frequency at high frequency stability such as a TCXO (Temperature Compensated Crystal Oscillator). The mobile equipment measures the transmission time interval of the control signal in response to the output of the low-rate clock oscillator, and estimates the reception timing. In addition, it brings the high-rate clock oscillator into operation only in proximity of the estimated desired reception timing to receive the control signal transmitted intermittently. With such a configuration, the mobile equipment enables the high-rate clock oscillator with rather large power consumption to operate intermittently in synchronization with the transmission timing of the control signal, thereby reducing the power consumption in an incoming call waiting state and the like.
As the low-rate clock oscillator used for the reception timing estimation, an oscillator with low power consumption and low frequency is usually selected. However, such an oscillator usually has a frequency deviation up to about 100 ppm. Thus, its frequency stability is vastly inferior to that of the high-rate clock oscillator of about a few parts per million. Therefore, to estimate the reception timing of the control signal accurately in response to the output of the low-rate clock oscillator, it is necessary to measure the frequency of the low-rate clock oscillator deviation in advance, and to correct the frequency deviation before estimating the reception timing in response to the low-rate clock signal after the correction.
FIG. 1 is a block diagram showing a configuration of mobile equipment employing a conventional frequency deviation estimating method of a low-rate clock signal, which is disclosed in Japanese patent application laid-open No. 2000-13269, for example. The operation of the conventional mobile equipment will be described.
First, a high-rate clock oscillator 1 generates an operation clock signal 102 with a high frequency and small frequency deviation, and supplies it to a timing controller 103 and to another received signal processing section to be used for the signal reception processing, not shown in FIG. 1. In contrast, a low-rate clock oscillator 2 generates a timing clock signal 109 with a low frequency and large frequency deviation.
A PLL 104, receiving the operation clock signal 102, increases its frequency by a specified scaling factor, and outputs a deviation measuring clock signal 105.
In response to a frequency deviation measuring instruction 113 of the timing clock signal 109 output from the timing controller 103, a measuring duration control counter 110 instructs the deviation measuring counter 106 to start counting the number of the clock pulses of the deviation measuring clock signal 105. At the same time, the measuring duration control counter 110 starts counting the number of clock pulses of the timing clock signal 109. The measuring duration control counter 110 has the number of clock pulses of the timing clock signal 109 for defining the duration of the frequency deviation measurement (called “deviation measuring clock number” from now on).
The deviation measuring clock number is set such that desired measurement accuracy is obtained as a result of the frequency deviation measurement. For example, to achieve the frequency deviation measurement at the accuracy of one millionth of the timing clock signal 109, a suitable deviation measuring clock number is set such that the frequency deviation measuring duration, which is defined by the period of the timing clock signal 109 and the deviation measuring clock number, becomes one million times the period of the deviation measuring clock signal 105.
Since the frequency of the deviation measuring clock signal 105 is multiplied by the PLL 104, the frequency deviation measuring duration can be reduced by increasing the multiplication number of the PLL 104. The power consumption of the PLL 104, however, increases with the multiplication number of the deviation measuring clock signal 105.
When the number of clock pulses of the timing clock signal 109 is counted up to the deviation measuring clock number, the measuring duration control counter 110 instructs the deviation measuring counter 106 to stop counting the number of the clock pulses of the deviation measuring clock signal 105, and notifies the timing controller 103 of the completion of the frequency deviation measurement.
Subsequently, the timing controller 103 reads the count value of the deviation measuring counter 106 as to the deviation measuring clock signal 105, and calculates the frequency deviation 115 of the timing clock signal 109 in response to the count value.
The calculated frequency deviation 115 of the timing clock signal 109 is supplied to a reset counter 114 for generating an estimated reception timing to be used for the frequency deviation correction of the timing clock signal 109.
With the foregoing configuration, the conventional radio receiver detects the frequency deviation of the timing clock signal 109 using the deviation measuring clock signal 105 the PLL 104 generates by multiplying the frequency of the operation clock signal 102, thereby reducing the frequency deviation measuring duration. Accordingly, it is necessary for the conventional radio receiver to include the PLL 104 for multiplying the frequency of the operation clock signal 102. This presents a problem of increasing the circuit scale and the power consumption for the frequency deviation measurement.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a radio communication apparatus and its reception timing estimating method capable of estimating the reception timing of the radio signal which is transmitted intermittently, at high accuracy by measuring the frequency deviation of the timing clock signal used for generating the estimated reception timing at high accuracy with curbing an increase of the power consumption.